This invention relates generally to electronically controlled fuel injection systems and, more particularly, to a method and apparatus for accurately delivering multiple separate fuel injections to the cylinder of an internal combustion engine during a fuel injection event based upon engine operating conditions when two fuel injection events are separated by a short period of time.
Electronically controlled fuel injectors are well known in the art including both hydraulically actuated electronically controlled fuel injectors as well as mechanically actuated electronically controlled fuel injectors. Electronically controlled fuel injectors typically inject fuel into a specific engine cylinder as a function of an injection signal received from an electronic controller. These signals include waveforms that are indicative of a desired injection rate as well as the desired timing and quantity of fuel to be injected into the cylinders.
Emission regulations pertaining to engine exhaust emissions are increasingly becoming more restrictive throughout the world including, for example, restrictions on the emission of hydrocarbons, carbon monoxide, the release of particulates, and the release of nitrogen oxides (NOx). Tailoring the number of injections and the injection rate of fuel to a combustion chamber, as well as the quantity and timing of such fuel injections, is one way in which to control emissions and meet such emission standards. As a result, split fuel injection techniques have been utilized to modify the burn characteristics of the combustion process in an attempt to reduce emission and noise levels. Split injection typically involves splitting the total fuel delivery to the cylinder during a particular injection event into two or more separate fuel injections, for example, a pilot injection and a main injection, or a main injection and an anchor injection, which may each be referred to generally as a fuel shot. As used throughout this disclosure, an injection event is defined as the injections that occur in a cylinder during one cycle of the engine. For example, one cycle of a four cycle engine for a particular cylinder, includes an intake, compression, expansion, and exhaust stroke. Therefore, the injection event in a four stroke engine includes the number of injections, or shots, that occur in a cylinder during the four strokes of the piston. The term shot as used in the art may also refer to the actual fuel injection or to the command current signal to a fuel injector or other fuel actuation device indicative of an injection or delivery of fuel to the engine. At different engine operating conditions, it may be necessary to use different injection strategies in order to achieve both desired engine operation and emissions control. In the past, the controllability of split injection has been somewhat restricted by mechanical and other limitations associated with the particular types of injectors utilized. Even with more advanced electronically controlled injectors, during certain engine operating conditions, it is sometimes difficult to accurately control fuel delivery, even when utilizing current control signals.
When dealing with split or multiple fuel injection and the general effects of a boot type fuel delivery and the fuel injection rate shaping which results therefrom, desired engine performance is not always achieved at all engine speeds and engine load conditions. Based upon operating conditions, the injection timing, fuel flow rate and injected fuel volume are desirably optimized in order to achieve minimum emissions and optimum fuel consumption. This is not always achieved in a split injection system due to a variety of reasons, including limitations on the different types of achievable injection rate waveform types, the amount of fuel injected during the pilot shot, when the two injections take place during the particular injection event, the timing sequence between the two injections, and how closely spaced injections influence each other. As a result, problems such as injecting fuel too rapidly within a given injection event and/or allowing fuel to be injected beyond a desired stopping point can adversely affect emission outputs and fuel economy.
In a system in which multiple injections and different injection rate waveforms are achievable, it is desirable to control and deliver any number of separate fuel injections to a particular cylinder so as to minimize emissions and fuel consumption based upon the operating conditions of the engine at that particular point in time. This may include splitting the fuel injection into more than two separate fuel shots during a particular injection event, (e.g., a pilot, main, and anchor injection), varying the fuel quantities in the shots, advancing the pilot shot during the injection event, and adjusting the timing between the various multiple fuel injections in order to achieve minimal emissions and desired fuel consumption. In some situations, it is also desirable to rate shape the front end of the fuel delivery to the cylinder to control the burn characteristics of the particular fuel being utilized.
When an injection event includes three distinct fuel injection shots, the delays between the individual fuel shots can become so short that the injection device does not have time to fully close during the delay period, and because the injection device is not completely closed when the subsequent fuel injection shot is supposed to begin, the amount of fuel actually injected deviates from the desired amount of fuel to be injected by a significant amount, e.g. as much as 30 cubic mils in some embodiments. This excess amount of fuel may adversely affect the efficiency and emissions benefits anticipated from the original fuel split.
Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.
In one aspect of the present invention, there is disclosed an electronically controlled fuel injection system which is capable of delivering multiple separate fuel injections to a particular cylinder of an internal combustion engine during a single injection event. The system includes at least one fuel injection device operable to deliver a plurality of fuel injection shots and a controller which is operable to determine a desired anchor duration based on a total main and anchor fuel quantity, a main shot duration, an anchor delay, and a rail pressure if the main and anchor fuel shots are closely coupled in time. Basing the anchor duration on these four variables achieves the injection of the desired fuel amount even when a boot condition occurs.
In a preferred embodiment, the controller is operable to determine that when both a main fuel shot and an anchor fuel shot are desired, then the main duration and the anchor delay may be determined by looking up these values in lookup tables or maps which use engine speed and load/fuel as factors in determining the appropriate main duration and anchor delay. The rail pressure is simply a reading taken from the rail manifold by a sensor, and the total main and anchor fuel quantity is found by subtracting a pilot shot fuel amount, if any, from the governor fuel output, also referred to as the total desired fuel quantity, or the total available fuel quantity, as requested by the governor. Before determining the anchor duration as a function of the five dimensional map, the program preferably confirms the occurrence of a triggering condition. In the illustrated preferred embodiment, a main shot fuel amount is compared with a minimum main shot fuel amount to determine if the main shot fuel amount is less than or equal to the minimum. If the main shot fuel amount is less than the minimum, then the anchor duration is set as a function of the four variables. Otherwise, control is returned to a main program.
In another aspect of the present invention, a computer readable medium contains instructions for controlling the fuel injection control system to set the anchor duration. The instructions determine four variables: rail pressure, total main and anchor fuel quantity, main shot duration, and anchor delay. The anchor duration is then determined based on those four variables.
In a preferred embodiment, the instructions determine the occurrence of a triggering condition. If the triggering condition has not occurred, program control is returned to a main program. If the triggering condition has occurred, then the anchor duration is determined via a five dimensional table or map using rail pressure, total main and anchor fuel quantity, main shot duration, and anchor delay as variables. Preferably, the triggering condition is when the main shot fuel amount is less than or equal to a minimum fuel amount, and a nonzero anchor fuel quantity is specified. In the preferred embodiment, the minimum main shot fuel amount can be set to a desired value. Alternatively, the minimum main shot fuel amount can be determined from a table or map as a function of engine speed and load/fuel. Thus, the minimum main shot fuel amount and a nonzero anchor fuel quantity provides a threshold for entry into a boot condition in which two closely coupled fuel injection shots lose their separate identity by running together. In one embodiment, the minimum main fuel amount is determined in a manner to be indicative of the threshold between a boot and a split injection, wherein, more fuel is generally injected in a boot condition that a split condition.
In still another aspect of the present invention, a method is described for controlling a fuel injection control system to determine an anchor duration. The method comprises sensing and transmitting a rail pressure to the controller, and determining a total main and anchor fuel quantity, a main shot duration and an anchor delay. The anchor duration is then determined based on the total main and anchor fuel quantity, main shot duration, anchor delay, and rail pressure. An injection signal indicative of the respective fuel shots including the anchor duration is then transmitted from an electronic controller to an injector.
In the preferred embodiment, the method includes determining the occurrence of a triggering condition and upon such occurrence, determining the anchor duration based on the total main anchor fuel quantity, main shot duration, anchor delay, and rail pressure. Upon the non-occurrence of the triggering condition, control is returned to a main program.